Traffic generated non-exhaust particulate emissions from concrete pavement: A mass and particle size study for two-wheelers and small cars

Abstract

This study aimed to understand the non-exhaust (NE) emission of particles from wear of summer tire and concrete pavement, especially for two wheelers and small cars. A fully enclosed laboratory-scale model was fabricated to simulate road tire interaction with a facility to collect particles in different sizes. A road was cast using the M-45 concrete mixture and the centrifugal casting method. It was observed that emission of large particle non exhaust emission (LPNE) as well as PM₁₀ and PM_2.5 increased with increasing load. The LPNE was 3.5 mg tire⁻¹ km⁻¹ for a two wheeler and 6.4 mg tire⁻¹ km⁻¹ for a small car. The LPNE can lead to water pollution through water run-off from the roads. The contribution of the PM₁₀ and PM_2.5 was smaller compared to the LPNE particles (less than 0.1%). About 32 percent of particle mass of PM₁₀ was present below 1 μm. The number as well as mass size distribution for PM₁₀ was observed to be bi-modal with peaks at 0.3 μm and 4–5 μm. The NE emissions did not show any significant trend with change in tire pressure.

Introduction

Identification and quantification of fugitive/non-point emission sources (emissions not released through stacks, vents, ducts or pipes) can be quite challenging. For these sources, emission factors are either not developed or, if developed, are associated with high amounts of uncertainty (Tucker, 2000). One of the important non-point sources is traffic related non-exhaust (NE) emissions. The NE emission consists of tire wear, road wear and brake wear. As the trend is towards cleaner exhaust gases through the use of catalytic converters and improved fuels and engines, soon particulate and other toxic emissions (e.g., polyaromatic hydrocarbons (PAHs)) from tire wear may surpass the exhaust emissions. According to EMEP/CORINAIR (2004), traffic-related NE sources in European countries contribute 3.1% and 1.7% of total PM₁₀ (particles having less than 10 μm diameter) and PM_2.5 (particles having less than 2.5 μm diameter), respectively. Thus, the understanding of generation and contribution of tire and road wear to particulate matter (PM) and carcinogenic compounds such as PAHs emissions acquires significance.

NE air-borne PM of size below 30 μm as well as above 30 μm is produced as a result of interaction between tires and road surface. Generation of frictional shear forces by the relative movement of surfaces is the main mechanism for the production of particulates. Tread wear rates have been reported to be between 0.008 and 0.09 g km⁻¹ per tire depending on driving conditions, roads, tire construction, vehicle load, etc. (Rogge et al., 1993, Luhana et al., 2004). Tire tread particles have been reported at considerable concentrations in different environmental compartments, such as rivers (Kumata et al., 1997, Kumata et al., 2000), sediments (Kumata et al., 2000, Voparil et al., 2004) and air (Kim et al., 1990, Allen et al., 2006).

PM above 30 μm gets deposited on the sides of the roads and contributes to urban run-off. Tread wear is a significant source of zinc (Adachi and Tainosho, 2004, Hjortenkrans et al., 2007) and PAHs (Pengchai et al., 2005, Boonyatumanond et al., 2007) to the environment. Several laboratory studies have shown water extracts of tires to be toxic to aquatic organisms (Gualtieri et al., 2005, Stephensen et al., 2005, Wik and Dave, 2009). A secondary mechanism involves the vaporization of material from road and tire surfaces due to the high temperatures developed by friction. This leads to the emission of highly carcinogenic volatile compounds such as the PAHs (KEMI, 2003). The studies in literature on estimation of NE emissions can be summarized into three categories:

• Determination of particle emissions by direct measurements using a simulated wheel in the laboratory (Williams and Cadle, 1978, Dahl et al., 2006, Gustafsson et al., 2008a, Kupiainen, 2007);

• Sampling and analysis of PM in ambient air followed by application of source apportionment methods (e.g., receptor modeling) (Rogge et al., 1993, Luhana et al., 2004); and

• Measurement of street dust in driving conditions using mobile units (Dannis, 1974, Hussein et al., 2008, Pirjola et al., 2008).

Studies (EMEP/CORINAIR, 2004, Luhana et al., 2004, ChemRisk, 2008) indicate that the emissions depend on the characteristics of the road surface (moisture content, type, etc.), tire and vehicle characteristics, and the driving cycle. In receptor modeling, variations in the above parameters are difficult to consider and modeling results may be applicable to a specific site only.

Laboratory set-ups that can simulate tire or road wear, require special skills, controls, safety measures, space and financial resources. Williams and Cadle (1978) collected and analyzed the emissions from tire runs in an enclosed indoor facility. They constructed a tire emission facility using a frame section and suspension assembly from an Oldsmobile Tornado and a continuous road surface using a steel pulley drum. The emission of gases and air-borne particulates was 1–20% (2–5 mg km⁻¹ for a car) of the total tire emissions; the balance of the emissions was large particles, which settled easily. The degradation of the tire tread during the wear process produced small quantities of gaseous hydrocarbons and sulphur compounds. The emission of the monomers and the dimers of styrene-butadiene rubber (SBR) indicated that local areas of the contact patch experienced temperatures high enough to degrade the polymer and volatilize the extender oils (Cadle and Williams, 1978).

Dahl et al. (2006) used the VTI (Swedish National Road and Transport Research Institute) road simulator to study the effect of studded and non-studded (friction) tires on different types of pavements. The studded tires have studs distributed on the tire surface to enhance the tire grip on icy road surfaces. These are used only in a few countries in the world. Non-studded winter tires have softeners added to the rubber to keep them soft under cold conditions and have more fringes to enhance the tire grip on the street during frozen street conditions. Dahl et al. (2006) found that the particles largely originate from the tires, and not the road pavement. A significant source of sub-micrometer fine particles was observed. The mean particle diameters (mean of the number distribution) in the ultra-fine region (<0.1 μm) were reported between 15 and 50 nm. They found that the emission factors for particles (number) originating from the road-tire interface were similar in magnitude to emission factors from liquefied petroleum gas-powered vehicles. Gustafsson et al., 2008a, Gustafsson et al., 2008b used VTI simulator to study the effects of different pavements, tire type and vehicle speed on pavement wear. The results showed that a granite pavement was more prone to PM₁₀ production than quartzite pavement. PM₁₀ from summer tires was negligible compared to studded and non-studded winter tires. The emphasis in this (Gustafsson et al., 2008a, Gustafsson et al., 2008b) study was on the effect of studded and non-studded tires on pavement wear and not on tire wear.

Hussein et al. (2008) used a mobile measurement system to study factors affecting non-tailpipe aerosol particle emissions from paved road. They observed that the highest particle mass concentrations were always observed behind the studded tire and the lowest were behind the summer tire regardless of the asphalt type. There was no clear speed dependence for the particle mass concentration behind the summer tires. The study also measured the re-suspended dust from roads in the experiments.

The other source of estimation for NE emission factors is from USEPA (2003). These factors have come under criticism (Venkatram, 2000, APEG (Airborne Particles Expert Group), 1999). The USEPA (2003) modeling approach considers road silt loading as the predominant source for particle emissions and assumes that most vehicle-related NE PM₁₀ arises from re-suspension. This is not true, as most of the emission could be released directly from road and tire wear. It needs to be recognized that re-suspended dust from road could have originated from other sources and, thus, the USEPA factor may over-estimate the emissions. The UK Airborne Particle Expert Group (APEG, 1999) considers the model unsuitable for UK climatic conditions. The MOBILE 6.1 tire wear emission estimates are used for air quality modeling. However, these factors have not been verified experimentally for existing or new pavement surfaces (Allen et al., 2006). The road characteristics can change the wear rate of the tire.

There are limited studies on tire wear on concrete pavements. Based on project-specific evaluation, at times, concrete pavements are advantageous over bituminous pavements (CAC, 2004). Concrete pavements have low rutting potential, durable skid resistance, longevity, better night-time visibility and heat reflecting characteristics. Recently, concrete pavements have been gaining prominence in India, especially in the areas that face heavy rains and are prone to water logging. This study evaluates the wear of tire and the particle size distribution for a concrete road as a function of various parameters for two-wheelers, three-wheelers and small cars (≤800 cc engine displacement).

Two wheelers are the most common mode of personal transport in India. Nearly 70% of the vehicles in Delhi (CPCB, 2008) and 83% in Kanpur (KMC, 2006) are two-wheelers. Three-wheelers (used as private taxis) are fewer in numbers (2–4%) but have a large kilometer travel in a day. The other major percentage comes from small cars. Hence the NE vehicular pollution would have a large contribution from such light vehicles in India and other developing countries. The study especially concentrates on the weight range of two and three-wheelers (engine displacement volume from 100 to 400 cc).

As seen from the literature review, NE emissions from concrete roads have not been studied extensively. Further, there are limited studies in the literature that have examined NE emission for two and three-wheelers. The objective of this research is to study particulate emissions through a parametric study with respect to tires (such as the load on the tire and the tire pressure) and concrete roads using a laboratory-scale physical model. The study concentrates on the use of summer tires commonly used in tropical countries that run on a concrete pavement.